In circuit design with transistors used as a power switch, the designer usually implements the power switch by using two transistors in a back-to-back series connection to prevent a parasitic diode inside the transistor (i.e., a base diode) from being turned on and causing a situation where the power switch cannot be completely turned off. However, because the two transistors may have different threshold voltages, when the power switch conducts a soft start, the two transistors may not be able to switch from a turned-off state to a turned-on state at the same time. If a time interval in which one of the transistors is switched from the turned-off state to the turned-on state only partially overlaps with a time interval in which the other transistor is switched from the turned-off state to the turned-on state, an overall equivalent resistance variation of the two transistor becomes unsmoothed. Consequently, during startup, the power switch will generate an inrush current, which leads to the soft start failure.